The present invention relates to a vacuum pump and, more particularly, to a turbomolecular pump for establishing a clean ultra-high vacuum.
A vacuum chamber having a high degree of vacuum has been required in nuclear fusion reactors, semiconductor production plants, electron microscope devices, etc. And, for this purpose, a so-called turbomolecular vacuum pump, exhibiting an excellent evacuation performance in a molecular flow region, has generally be adopted therefor.
While the performance of conventional turbomolecular vacuum pumps has been up to approximately 10.sup.-10 Torr in terms of the degree of vacuum, it has been desired to achieve a still higher degree of ultra-high vacuum due to the enhanced performances of various apparatus to which the vacuum pump may be applied. Moreover, not only is it desirous to bring the degree of vacuum to a ultra-high vacuum, it is also necessary to attain a clean ultra-high vacuum in which a hydrocarbon residual gas such as, for example, oil vapor does not exist.
In principle, considering the mechanism and operation of a turbomolecular pump, a great compression ratio can be set for the oil vapor or the like having a large molecular weight in a gas stage, and such substance is emitted well so that a realization of a clean vacuum is basically possible.
However, in many conventional turbo molecular vacuum pumps, it is necessary to provide a ball bearing arrangement of a mechanical contact type for a rotor of the pump in order to enable a high speed rotation and, consequently, it is necessary that a bearing oil be supplied to the bearing for the purpose of lubrication thereof. Although a turbomolecular vacuum pump employing the ball bearing performs tentative evacuation to attain a clean vacuum during operation, the rotating ability is limited due to, for example, vibrations, etc. Additionally, once the turbomolecular vacuum pump has stopped operating, the oil vapor of the lubricating oil of the bearings may diffuse in the reverse direction to the high vacuum side thereby staining the vanes of the pump and the vacuum chamber.
In order to alleviate the above noted problems, a turbomolecular vacuum pump has been proposed in, for example, U.S. Pat. No. 4,023,920, wherein all bearings are fashioned as controlled magnetic bearings. However, a disadvantage of this proposed vacuum pump resides in the fact that complicated mechanisms such as control devices, etc., are required. Additionally, in this proposed vacuum pump the control is intricate and the maintenance and inspection thereof are relatively difficult. A further disadvantage resides in the fact that a turbomolecular vacuum pump with controlled magnetic bearings is very expensive to manufacture.
In, for example, U.S. Pat. No. 4,111,595, a vacuum pump arrangement is proposed wherein magnetic bearings arranged on a high vacuum side thereof are in the form of a permanent-magnet. While the provision of a permanent-magnet bearing dispenses with the need for complicated control mechanisms and, to some extent, reduces the overall manufacturing costs, a disadvantage of this proposed construction resides in the fact that a permanent magnet is usually made of a sintered metal which has a large quantity of emission gas when compared with stainless steel or the like. thus, the problem arises that the influence of the gas emitted from the permanent magnet arranged on the high vacuum side increases as the degree of vacuum rises to limit the arrival degree of the vacuum pump.